Cryptography – Communication system using cryptography – Wireless communication
Patent
1996-05-14
1998-07-07
Gregory, Bernarr E.
Cryptography
Communication system using cryptography
Wireless communication
380 9, 380 28, 380 40, H04K 102, H04K 110, H04L 900
Patent
active
057780730
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND
1. Field of the Invention
The present invention relates to methods and apparatus for the encryption and decryption of speech in voice transmission. More particularly, the invention pertains to voice transmission apparatus of the type that includes a front-end unit for digitizing a voice signal and matching a transmitted signal to a predetermined transmission channel and for digitizing a received signal and matching the conditioned received signal to a voice reproduction device.
2. Description of the Prior Art
Reference will be made in the discussion that follows to the methods listed immediately below that relate to the prior art for voice encryption and decryption. transmission as digital data using a MODEM. a plurality of smaller time intervals, transmission of such sub-sequences in a sequence other than the original. transmission of a signal which is produced by interchanging spectral sub-bands. of the audio-frequency spectrum to be transmitted using a fixed or variable splitting device (mirror-image frequency method).
The above known methods are subject to the following fundamental disadvantages:
Regarding 1) As a rule, the same channels must be employed for transmission of the digital data as for unencrypted speech. Since such channels are of limited bandwidths, data reduction methods are required. After reconstruction of the (reduced) data at the receiving end, it is impossible to identify the person speaking reliably.
Regarding 2) For physiological reasons, the number and time duration of the sub-interval can only be varied within strict limits, simplifying decoding of the transmitted signal.
The transitions between interchanged sub-intervals generally cannot be reconstructed in proper phase at the receiving end. As a result, a noticeable reduction in signal quality is heard when compared to the unencrypted signal.
A fundamental, perceptible delay exists between speech and signal transmission that can lead to disturbing echo effects in certain types of transmission channels.
Regarding 3) The number and bandwidths of the spectral sub-intervals are strictly limited due to physiological considerations, making decoding easy. Unavoidable bandwidth overlaps of the filters required for production and reconstruction of the sub-spectra lead to a deterioration in transmission quality.
Regarding 4) Decoding of the transmitted signal can be done with a relatively minor technical investment. The residual comprehensibility of the encrypted signal is high; trained listeners can monitor transmissions without technical aids.
Regarding 5) Combinations of the various methods generally enhance security against decryption. Unfortunately, they also lead to accumulation of such undesirable properties as the deterioration of signal-to-noise ratio and limitation to a small number of simple constellations of transmission channels.
SUMMARY AND OBJECTS OF THE INVENTION
It is therefore an object of the present invention to create a method and apparatus for speech encryption and decryption of voice transmission suitable for production as a compact module that can be retrofitted.
It is a further object of the invention to achieve the above object while obtaining improved security against monitoring and evaluation by third parties over that offered by known methods and devices.
Other objects of the invention include achieving the above-stated object while obtaining good speech comprehensibility and voice recognition, little variance in quality from clear operation, operation and controllability that are largely transparent to the user, automatic recognition of encrypted signals at the receiving end, capability of use in analog radio networks and in the telephone field and conformity with the specified available transmission bandwidths.
The preceding and other objects are addressed by the present invention which provides, in a first aspect, a method for speech encryption and decryption of a voice transmission. Such method is begun by converting the digitized voice signal c(.nu.) into a complex signal x(n) at a transm
REFERENCES:
patent: 5048086 (1991-09-01), Bianco et al.
patent: 5245660 (1993-09-01), Pecora et al.
patent: 5291555 (1994-03-01), Cuomo et al.
patent: 5379346 (1995-01-01), Pecora et al.
Patent Abstracts of Japan; vol. 13, No. 109; App. No. JP870112620, App. Date Nov. 5, 1987.
Busching Wolfram
Schlenker Erhard
Spahlinger Gunter
Gregory Bernarr E.
Kramsky Elliott N.
Litef GmbH
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